Communication apparatus including driver means for applying a switched signal to a communication line with a controlled slew rate

ABSTRACT

A communication node including drivers for applying a switched signal to a communication line such as a CAN bus or a LIN bus, with a controlled slew rate. The driver comprises a series of transfer elements and a series of delay elements for cumulatively establishing operational connections of the transfer elements with the communication line, whereby to apply the switched signal progressively to the communication line. A feedback loop is responsive to the signal that the driver applies to the communication line for controlling the delays of the delay elements so as to control the delays with which the operational connections of the transfer elements with the communication line are established.

FIELD OF THE INVENTION

This invention relates to communication apparatus including driver meansfor applying a switched signal to a communication line with a controlledslew rate.

BACKGROUND OF THE INVENTION

Local networks often make use of a communication line, such as acommunication bus, over which a set of nodes communicates. A drivermodule in a master node applies power to the line, the driver modulebeing switched to produce step changes in the power in the line totransmit signals to receivers in remote slave nodes over the line. Theswitched power signal activates the multiplexed remote nodes connectedto the line and the line also selectively transmits signals from theremote nodes back to a central processing unit.

Such a bus is used in automotive vehicles, for example, the bus eithercomprising a single line or often comprising a twisted pair ofconductors in which the current flows, the close coupling between thepair of conductors reducing their sensitivity to electromagneticinterference (‘EMI’), that is to say reception of noise induced in thewires of the bus, and improving their electromagnetic compatibility(‘EMC’), that is to say the radiation of parasitic fields by thecurrents flowing in the wires of the bus; both are critical parameters,especially in automotive applications.

Historically, in automotive applications, functions such as door locks,seat positions, electric mirrors, and window operations have beencontrolled directly by electrical direct current delivered by wires andswitches. Such functions may today be controlled by ECUs (ElectronicControl Units) together with sensors and actuators in a multiplexedController Area Network (CAN). The Controller Area Network (CAN)standard (ISO 11898) allows data to be transmitted by switching avoltage, at a frequency of 250 kbauds to 1 Mbaud for example, to themultiplexed receiver modules over the twisted pair cable. The receivermodules may be actuators that perform a function, for example bygenerating mechanical power required, or sensors that respond toactivation by making measurements and transmitting the results back tothe ECU over the bus.

The CAN bus was originally designed to be used as a vehicle serial databus, and satisfies the demands of real-time processing, reliableoperation in a vehicle's EMI environment, is cost-effective, andprovides a reasonable data bandwidth. A variant on the CAN standard isthe LIN (Local Interconnect Network) sub-bus standard (see ISO 7498),which is an extension to the CAN bus, at lower speed and on a singlewire bus, to provide connection to local network clusters.

The wires of a communication bus are often long and present asubstantial distributed reactive load to the transmitter to which theyare connected and especially their capacitive loads may be individuallyvariable. It is important, for example for meeting acceptable EMI andEMC performance levels, that the slew rate of the switched signals (thatis to say their rate of rise and fall of amplitude) is controlled inorder to enable precise timing of certain elements of the signalstransmitted. In particular, in the case of a bus comprising a pair ofconductors, matching of the slew rate between the two conductors isimportant.

SUMMARY OF THE INVENTION

The present invention provides communication apparatus as described inthe accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified generic schematic diagram of a Controller AreaNetwork node comprising a driver and a receiver connected to a pair ofCAN bus lines,

FIG. 2 is a waveform diagram of signals appearing in operation of theCAN node of FIG. 1,

FIG. 3 is a schematic diagram of a known driver in a CAN node of thekind shown in FIG. 1,

FIG. 4 is a schematic diagram of a CAN node of the kind shown in FIG. 1in accordance with one embodiment of the invention, given by way ofexample,

FIG. 5 is a simplified circuit diagram of a delay element in the CANnode of FIG. 4,

FIG. 6 is a simplified circuit diagram of a delay sub-element in thedelay element of FIG. 5 and

FIG. 7 is a simplified circuit diagram of a driver transfer module inthe CAN bus of FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows, by way of example, the general principle of a CAN bussystem, the CAN system comprising a number of nodes, FIG. 1 showing oneof the nodes comprising a driver 1 and a receiver 2, the nodescommunicating over a pair of bus lines 3 and 4, the bus line 3 being theCANH line and the line 4 being the CANL line. In accordance with the CANspecifications, any one of the nodes may be a master node and anyone maybe a slave node, according to the communication to be transmitted.

More particularly, the driver 1 comprises first and second driverelements 5 and 6 connected to receive an input signal Tx fortransmission over the bus lines 3 and 4, the assertion of the signal Txswitching the driver 5 to connect the CANH line 3 to a voltage source 7at a voltage VCC and the driver 6 to connect the CANL line 4 to a groundterminal 8.

The receiver 2 comprises a differential input 9 that is responsive tothe difference in voltage between the CANH line 3 and the CANL line 4 toproduce a digital output signal Rx at a terminal 10. It will beappreciated that, when the node is the master node, the driver 1 isactive to transmit signals to remote nodes over the bus lines 3 and 4and, when the node is a slave node, the receiver 2 is active to receivesignals from other nodes over the bus lines 3 and 4.

FIG. 2 shows the signals appearing in a substantially ideal case inoperation of the system of FIG. 1. The input signal Tx applied to theinputs of drivers 5 and 6 is a step signal. However, the bus lines 3 and4 present substantial distributed impedances including reactivecomponents, so that the signal 11 transmitted over the CANH bus line 3and the signal 12 transmitted over the CANL bus line 4 are not stepsignals but exhibit rising edge and falling edge slew phases 13 and 14in which the signal values change progressively. The receiver of aremote node reacts when the difference between the CANH signal 11 andthe CANL signal 12 applied to its input reaches a threshold value on therising edge and on the falling edge and accordingly, the output signalRx exhibits its step changes with delay times 15 and 16 relative to thestep changes in the input signal Tx.

In the ideal case shown in FIG. 2, the slew rates during the rising andfalling slew phases 13 and 14 are identical, so that the delay times 15and 16 of the corresponding step changes in the output receiver signalRx are identical. Moreover, ideally, the signals flowing in the CANHline 3 and CANL line 4 are always equal and opposite with identicaltiming and rates of change so that the combined signal CANH+CANLpresented to the external world is always zero, theoretically reducingthe electromagnetic emissions to zero. However, in practice, thedistributed impedances of the lines 3 and 4 are individually variableand good matching between the high side driver 5 and the low side driver6 is difficult to achieve. Accordingly, especially during the slewphases 13 and 14, the common mode signal CANH plus CANL is not constantand even equality between the slew rates of the rising edge and thefalling edge cannot be guaranteed.

A first improvement in the performance of the system may be obtained bythe use of drivers which comprise respective series of transfer elementsand delay elements for cumulatively establishing operational connectionsof the transfer elements with the bus lines 3 and 4 so as to apply thestep changes of the input signal Tx progressively to the bus lines 3 and4 during the slew phases 13 and 14. As shown in FIG. 3, a series of Nswitch elements 17 are connected between the VCC terminal 7 and the CANHline 3 through a series of N resistive elements 18. A correspondingseries of (N−1) delay elements 21 are connected in series so as totrigger respective switch elements 17 with delays in time defined by thecumulative effect of the delay elements 21. Similarly a series of (N−1)delay elements 22 is connected to trigger the switch elements 19. Theswitch elements 17 and 19 have fast switching times.

In operation, during the slew phases 13 and 14, the step changes in theinput signal Tx are progressively asserted and de-asserted on the lines3 and 4 by the cumulative connection in parallel of the resistiveelements 18 and 20. In the example shown in FIG. 3, the number N ofswitch elements and resistive elements is equal to 10, so that a stepchange in the signal Tx is applied in ten small steps to the bus lines 3and 4 but it will be appreciated that a greater or smaller number ofswitch elements and resistive elements may be used, as required. Thevalues of the resistive elements 18 and 20 are each N times the valuethat would be required for a single resistive element to apply the wholesignal Tx to the bus lines 3 and 4 respectively.

This system presents the advantage that the slew rate during the slewphases 13 and 14 now is controlled primarily by the cumulative delaytimes of the delay elements of the series 21 and 22. However, the delaytimes of the delay elements of the series 21 and 22 are fixed and, evenif the slew rates of the CANH line 3 and the CANL line 4 are now bettermatched, the slew rates are a function of temperature and of thecapacitive load of the CANH line 3 and the CANL line 4, so that the slewrates are still difficult to control

FIG. 4 shows a CAN system in accordance with one embodiment of thepresent invention, by way of example. The driver 1 again comprises twoseries of switch elements 17 and 19 switching two series of resistiveelements 18 and 20 to apply the Tx signal progressively through theresistive elements 18 and 20 to the bus lines 3 and 4. However, theseries of delay elements 21 and 22 are replaced by series of delayelements 23 and 24 respectively, the individual delay times of each ofthe delay elements of the series 23 and 24 being controlled by afeedback loop. More particularly, the feedback loop comprises a delayselection circuit 25, which increments or decrements delay times of thedelay elements as a function of the signal that the drivers 5 & 6 applythrough resistive elements 18 and 20 to the bus lines 3 and 4.

More particularly, the feedback loop comprises a reference signalgenerator 26 comprising a constant current source 27 and a calibratedcapacitor 28 and a switch 29 triggered by the step changes in the inputsignal Tx to trigger the charging of the capacitor 28. The voltage onthe capacitor 28 is compared with a reference voltage on a terminal 30by a reference receiver 31, similar to the signal receiver 2, and thesignal at the output 32 of the reference receiver 31 exhibits a stepchange when the voltage on the capacitor 28 exceeds or drops below thereference voltage on the terminal 30.

The outputs of the signal receiver 2 and the reference receiver 31 areapplied to a phase comparator 33 that generates an output that is afunction of the relative time delays between the step changes in thesignals on the outputs of the two receivers. The output of the phasecomparator 33 is supplied through a digital filter 34, which removesparasitic signals, to the delay selection circuit 25. The delayselection circuit 25 decrements or increments by unit amounts the delaysof the delay elements of the series 23 and 24 in response to an advanceor retard of the output of the signal receiver 2 relative to the outputof the reference receiver 31.

In operation, at the first step change in the input signal Tx, inresponse to any time difference between the step changes of the outputof the signal receiver 2 and of the reference receiver 31, the delayselection circuit 25 will increment or decrement the delay times of thedelay elements of the series 23 and 24, the operation being repeated atany subsequent step changes in the input signal Tx, so as to synchronisethe step changes of the signal receiver 2 and the reference receiver 31after a small number of cycles. The slew rates of the signals applied tothe bus lines 3 and 4 are thus controlled during the slew phases 13 and14 so that both the slew rate and timing of the signals is controlled.The slew time of the signals is substantially independent of the loadson the bus lines 3 and 4 and of the effect of temperature on the circuitelements.

FIG. 5 shows the structure of each of the (N−1) delay elements of theseries 23 and 24. Each of the delay elements itself comprises a set ofdelay sub-elements 36 connected in series to trigger each other insuccession. The outputs of each of the delay sub-elements of the set 36are connected through a respective multiplex switch element of a set 37to an output terminal 38. The signal from the delay selection circuit 25is applied through a connection 35 to select one of the switches of theset 37.

In operation, when a step change is applied to the input of the delayelement, the step change propagates through the set of delaysub-elements 36 until it reaches the selected switch of the set 37, whenthe step change is asserted on the output terminal 38, with a delay thatis a function of the position of the selected switch. In the preferredembodiment of the invention, the number M of sub-elements is equal to10, although a greater or lesser number may also be used.

FIG. 6 shows a preferred embodiment of a delay sub-element in the set 36and comprises an inverter 39 that receives the signal from the previousdelay element or, in the case of the first delay element of the series,the signal Tx. The output of the inverter 39 is connected to charge acapacitor 40 connected to ground and the voltage across the capacitor 40is supplied to a further inverter 41 whose output is connected to thecorresponding switch element of the set 37 and, except in the case ofthe last delay sub-element in the set, to the following delaysub-element. It will be appreciated that this structure offers a readyimplementation for the controllable delay element, using simpleelementary delay sub-elements and a multiplex arrangement.

Although the driver shown in FIG. 4 has separate series of delayelements 23 and 24 for the CANH and CANL sides, it is possible to use asingle series to control the series of switch elements 17 and 19 andtheir resistive elements 18 and 20. As shown in FIG. 7, each modulecomprises an input terminal 42 that is connected to the output terminalof the corresponding delay element, except in the case of the first ofthe series, for which it is connected to receive the input signal Tx.The input terminal 42 is connected through an inverter 43 to a switchelement 44 that is connected in series between the VCC terminal 7 andthe resistive element 45 in the series 18 that is connected to the CANHbus line 3. The input terminal 42 is also connected to a switch element46 that is connected in series between a resistive element 47 that ispart of the series 20 and the CANL bus line 4.

1. Communication apparatus including driver means for applying aswitched transmission signal to a communication line with a controlledslew rate, said driver means comprising a series of transfer elementsand delay means comprising a series of delay elements for introducingrespective individual delays in signals applied thereto and forcumulatively establishing operational connections of said transferelements with said communication line with respective cumulative delaysin time, whereby to apply said switched transmission signalprogressively to said communication line, wherein said delay meanscomprises feedback means responsive to said switched transmission signalthat said driver means applies to said communication line forcontrolling the individual delays introduced by said delay elementsrespectively, so as to control the cumulative delays with which saidoperational connections of said transfer elements with saidcommunication line are established.
 2. Communication apparatuscomprising driver means for applying a switched transmission signal to acommunication line with a controlled slew rate and a signal receiverconnected to respond to signals on said communication line, said drivermeans comprising a series of transfer elements and delay meanscomprising a series of delay elements for introducing respective delaysin signals applied thereto and for cumulatively establishing operationalconnections of said transfer elements with said communication line withrespective delays in time, whereby to apply said switched transmissionsignal progressively to said communication line, wherein said delaymeans comprises feedback means responsive to said switched transmissionsignal that said driver means applies to said communication line forcontrolling the delays of said delay elements, so as to control thedelays with which said operational connections of said transfer elementswith said communication line are established and wherein said feedbackmeans comprises reference means for generating a reference rate signalhaving a reference rate of change, said reference means being arrangedto start generation of said reference rate signal at a start of a slewphase of said switched signal, reference receiver means responsive to avalue of said reference rate signal relative to a reference signal, andcomparator means responsive to relative times of response of said signalreceiver and said reference receiver.
 3. Communication apparatus asclaimed in claim 2, wherein said driver means is arranged to applyrespective first and second switched transmission signals to a pair ofsaid communication lines, said comparator means being responsive torelative times of response of said signal receiver to said first andsecond switched transmission signals and of the signal from saidreference receiver means.
 4. Communication apparatus as claimed in claim1, wherein each of said delay elements comprises a series of delaysub-elements connected to trigger each other in succession, and saidfeedback means is arranged to select which of said delay sub-elements insaid series triggers establishment of an operational connection of thecorresponding one of said transfer elements with said communicationline.
 5. Communication apparatus including driver circuit for applying aswitched signal to a communication line with a controlled slew rate,said driver circuit comprising a series of transfer elements and delaycircuit comprising a series of delay elements for cumulativelyestablishing operational connections of said transfer elements with saidcommunication line, whereby to apply said switched signal progressivelyto said communication line, characterised in that said delay circuitcomprises a feedback circuit responsive to the switched signal that saiddriver circuit applies to said communication line for controlling theindividual delays of said delay elements, so as to control thecumulative delays with which said operational connections of saidtransfer elements with said communication line are established. 6.Communication apparatus as claimed in claim 5, and comprising a signalreceiver connected to receive signals from said communication line,wherein said feedback circuit comprises reference circuit for generatinga reference rate signal having a reference rate of change, saidreference circuit being arranged to start generation of said referencerate signal at the start of a slew phase of said switched signal,reference receiver circuit responsive to the value of said referencerate signal relative to a reference signal, and comparator circuitresponsive to relative times of response of said signal receiver andsaid reference receiver.
 7. Communication apparatus as claimed in claim6 for applying switched signals to a pair of said communication lines,wherein said driver circuit is arranged to apply respective first andsecond switched signals to the communication lines of said pair, saidcomparator circuit being responsive to relative times of response of acombined value of said first and second signals and of the signal fromsaid reference receiver circuit.
 8. Communication apparatus as claimedin claim 5, wherein each of said delay elements comprises a series ofdelay sub-elements connected to trigger each other in succession, andsaid feedback circuit comprises circuits for selecting which of saiddelay sub-elements in said series triggers establishment of anoperational connection of the corresponding one of said transferelements with said communication line.